Flame retardants are chemical additives which may be used across a variety of consumer products, such as plastics, textiles, leather, paper, rubber, etc. Chemicals which may be used as flame retardants can be mineral, halogen containing, nitrogen containing and phosphorus containing chemicals, silicon based chemicals, etc. The term “retardant” represents a class of use and not a class of chemical structure.
Preventive flame protection, including the use of flame retardants, has been practiced since ancient times. For example, alum was used to reduce the flammability by Egyptians at the time of about 540 BC. The advent of synthetic polymers earlier last century was of special significance, since the water soluble inorganic salts used up to that time were of little or no utility in these largely hydrophobic materials. Modern developments were, therefore, concentrated on the development of polymer compatible flame retardants. Wild forest fires comprise a serious problem, burning thousands of hectares all over the world each year. Diammonium phosphate (DAP), monoammonium phosphate (MAP), ammonium polyphosphate (APP) and ammonium sulphate (AS) have been used as long-term flame retardants. They are regarded as long-term flame retardants, because they can inhibit combustion even after the loss of their water matrix.
Fundamentally, four processes are involved in polymer flammability: preheating, decomposition, ignition and combustion and propagation. Flame retardants interfere with combustion during a particular stage of this process, i.e. during heating, decomposition, ignition or flame spread through physical or chemical actions.
There are several ways in which the combustion process can be retarded by physical action: for example cooling, formation of a protective layer/coating and/or dilution. During cooling action endothermic processes triggered by flame retardants may cool the material to a temperature below that required to sustain the combustion process. By formation of a protective layer/coating, a condensed combustible layer may be shielded from the gaseous phase with a solid or gaseous protective layer. A condensed phase is thus cooled, smaller quantities of pyrolysis gases are evolved, the oxygen necessary for the combustion process is excluded and heat transfer is impeded. By dilution, the incorporation of inert substances (e.g., fillers) and additives that evolve inert gases on decomposition may dilute the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not exceeded.
Flame retardants may impede combustion by providing chemical reactions which interfere with combustion processes occurring in the solid and/or gas phases. For reactions in the gas phase, a free radical mechanism of a combustion process which takes place in the gas phase is interrupted by a flame retardant. Exothermic processes may thus be stopped, the system cools down, and the supply of flammable gases is reduced and eventually completely suppressed. For reactions in the solid phase, two types of reaction may take place. Firstly, breakdown of a polymer may be accelerated by a flame retardant, causing pronounced flow of a polymer and, hence, its withdrawal from the sphere of influence of the flame, which breaks away. Secondly, a flame retardant may cause a layer of carbon to form on a polymer surface. This can occur, for example, through the dehydrating action of the flame retardant generating double bonds in the polymer. These may form a carbonaceous layer by cyclizing and cross-linking.
In recent years, there are growing concerns about the safety of these flame retardant chemicals. An issue with the above mentioned forest flame retarding chemicals are their impact on the environment. Initially it was thought that these flame retardants would have no adverse on the environment, as their main active ingredients are agricultural fertilizers. However, ammonia, coming from the dissociation of the ammonium salts, is regarded as extremely toxic. Ecotoxicological studies were performed to understand the effects of long-term forest fire retardants on enzymatic activities, cells and microorganisms, thereby obtaining LC50 levels (lethal concentration). The LC50 value of ammonia is 0.53-4.94, which is extremely toxic. Toxicity studies on aquatic organisms relate the results obtained to the determined amount of flame retardants and ammonia. The data show that ammonia is the component that has most impact on these organisms under the testing conditions.
Brominated flame retardants, such as polybrominated diphenylethers (PBDEs), were first introduced into the consumer marketplace in the 1970s. They showed great compatibility with plastics and textiles, and offered superior flame retardant properties. Brominated flame retardants interrupt combustion by volatizing bromine radicals to react with high energetic free radicals O. and .OH from the combustion, thereby preventing the spread of the flame. The most commonly used brominated flame retardants are PBDEs and tetrabromobisphenol A (TBBPA). By 2010, the brominated flame retardants market is projected to reach 1.7 billion pounds. Market Report by Peter Dufton; 2003.
Great efforts are being put into developing halogen free flame retardants, especially phosphorus based flame retardants. However, their flame retarding performance is not satisfactory. The prior art describes the use of some phosphonium ion salts. Doring et al describe polyphosphonium cations with selected anions as flame retardants in application US20100160476. Japanese patent application JP 2010163396 describes straight chain alkylaryl phosphonium salt structures as polymer dopants for high conductivity, heat resistance and flame retardancy. Tan et al, Faming Shuanli Shenqing Gongkai Shuomingshu, CAPLUS AN 2010:740737 (Patent), Jun. 9, 2010, reported fireproofing agent containing quaternary phosphonium salt-modified montmorillonite as flame retardants. A review by Guo et al, Zhongguo Pige, 2004, CAPLUS AN 2005:551561 describes development and applications of tetrakis(hydroxymethyl) phosphonium salts as flame retardants for textiles, leather tanning agents, bactericides for wastewater treatment, etc. Ammonium surfactants have been employed to modify the surface of nanoclays for flame retarding applications.
Despite health and environmental concerns, the world flame retardant chemicals market is projected to reach 5.7 billion pounds by the year 2012. The United States is the country with the tightest flame safety standards, and consequently the greatest use of brominated flame retardants. Nearly 98 percent of roughly 8,500 metric tons of PBDE used globally is consumed in US. However, brominated flame retardants are not chemically bound to the textiles and many substrates in plastic composites; therefore, they may easily escape into environment. There is growing concern over the persistence and bioaccumulation of brominated flame retardants and their risk to the environment and human health. Since brominated flame retardants are lipophilic and bioaccumulative substances, they may build up in fatty tissues once they enter a human or animal body. Studies have found bromated flame retardants to be widespread in the environment and in human tissues. Studies also have shown that these brominated flame retardants are toxic and can cause serious health disorders. In addition, women in North America have the highest levels globally of these chemicals in their breast milk.
The foregoing examples of the related art and limitations are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings or figures as provided herein.